This invention relates to in situ measurements of earth formations traversed by a well borehole. More particularly, the invention relates to the measurement of the thermal neutron decay time (or neutron lifetime) of earth formations in the vicinity of a wellbore.
The observed decay rate of the thermal neutron population in the vicinity of a well borehole following a pulse or burst of high energy neutrons can be approximated by the sum of formation and borehole exponential terms plus a background term which can vary according to formation and borehole conditions. In typical field conditions the borehole component of the thermal neutron lifetime, or decay time, decays more rapidly than the formation component of thermal neutron lifetime. The primary parameter of interest is .tau..sub.F the mean lifetime of thermal neutrons within the formation. Another parameter of interest is .tau..sub.B, the mean lifetime of thermal neutrons in the borehole.
The system and methods of the present invention employ a pulsed source of fast neutrons. The fast neutrons are slowed down (or moderated) to thermal energy rapidly by interaction with the nuclei of the elements in the borehole, the earth formations surrounding the borehole, and fluids contained in the pore spaces of such formations. The thermal neutron lifetime or decay time of the earth formation is largely determined by the salt or chlorine content of the earth formations. The hydrogeneous matter in the pore spaces and borehole rapidly attenuates or slows down the fast neutron flux emitted by a source of pulsed fast neutrons. The fast neutrons when slowed to thermal energy are said to be thermalized and may then be captured by the nuclei of elements comprising the formation matrix and fluids filling the formation matrix and the materials comprising the wellbore, including the borehole fluid, logging instrument, and possibly well casing. The element chlorine, which is found in highly saline borehole fluids and earth formation fluids in the pore spaces of earth formations in the vicinity of a borehole when a high salt (NaCl) content is present, has a very high capture cross-section for thermalized neutrons. Thus a measurement of the thermal neutron decay time or neutron lifetime of earth formations in the vicinity of a well borehole can be indicative of the amount of saline fluids present in the pore spaces of the formation. When combined with formation water salinity, porosity measurements and measurements of formation shaliness, this results in a combination which can be used to discriminate oil from salt water filled pore spaces in the vicinity of a well borehole.